In U.S. Pat. No. 5,137,774, the increased performance of .alpha.-Al.sub.2 O.sub.3 as compared to .kappa.-Al.sub.2 O.sub.3 as a coating on a cemented carbide insert when turning cast iron was shown. In addition, in U.S. Pat. Nos. 5,635,247 and 5,700,569 as well as U.S. Pat. No. 6,015,614 there are shown various alumina-coated cemented carbide inserts in which the alumina is deposited on a Ti(C,N) layer or multilayers. However, in tests as conducted by the present inventor, it was noticed that the adhesion of an .alpha.-Al.sub.2 O.sub.3 layer to the underlying TiCN layer as well as the adhesion of the TiCN layer to the cemented carbide substrate was often unsatisfactory when the insert was used in the turning of cast iron. The coating failed due to edge chipping which resulted in accelerated wear.
The main reasons for edge chipping have been identified by the present inventor from these tests to be the weak substrate-coating adhesion as well as the weak bond between TiCN and .alpha.-Al.sub.2 O.sub.3.
In one study of the TiC-cemented carbide interface of a 6 .mu.m thick CVD-deposited TiC layer by Vuorinen et al., "TEM Study of Microstructure and Crystallography at the TiC/Cemented Carbide Interface", Science of Hard Materials, 1983, pp. 433-447, it was found by transmission electron microscopy (TEM) that the TiC layer is composed of two regions. Close to the substrate and extending to a thickness of 1.5-2 .mu.m is a layer of fine, equiaxed TiC grains. Above that is a layer of larger (typically 2-4 .mu.m) grains of TiC.
In another study published in Thin Solid Films, 232 (1993) pp. 73-82, Vuorinen et al., entitled "Interfacial Characterization of Chemically Vapour Deposited Titanium Carbide on Cemented Carbide", TiC coatings were CVD-deposited on cemented carbide substrates under non-carburizing conditions. In the absence of .eta.-carbide, it was found that the TiC nucleated and grew epitaxially on both {0001}- and {1010}- WC planes.
Because of the increased performance in turning cast iron which is obtained when using a coated cemented carbide insert having an outer layer of .alpha.-Al.sub.2 O.sub.3, the search for acceptable inserts has continued.